1. Field of the Invention
This invention relates to semiconductor device packages and, more particularly to encapsulating integrated circuits (ICs) or discrete devices in such packages.
2. Discussion of the Related Art
In the semiconductor device industry it is common to fabricate device packages from a metallic base and plastic or other polymer sidewalls. One of the many potential package types is the known as an open cavity package, which is commonly used in applications where high thermal loads must be handled including, but not limited to, power devices (e.g., radio frequency, laterally diffused MOSFETS or RFLDMOSFETs). The most common open cavity package includes a high thermal conductivity base, ceramic side walls and imbedded leads. These packages are typically of the hermetic or semi-hermetic variety. In both varieties the semiconductor device or chip is connected to the base and the leads, and the chip is protected from the outside environment by a substantially leak tight, sealed lid. As such, there is no requirement for a semiconductor device encapsulant for environmental protection. For lower cost applications, the ceramic-side walls of the package can be replaced with plastic. Many of the open cavity plastic packages are non-hermetic by design. In this case the semiconductor device must be encapsulated so that unwanted environmental degradation does not occur. The best environmental protection is achieved when the entire cavity (including the semiconductor device, wire bonds, package leads, and package base) is filled with a protective encapsulant (typically in the form a cured gel, such as silicone). However, gel-like encapsulants form a meniscus when cured. Because the meniscus is naturally a depression in the central region of the gel layer, often not all portions of the wires or the chips are completely covered by gel in that central region.
In addition, before the gel is cured, it tends to wick up the side walls of the package and sometimes spills over the top of the package. This problem is exacerbated in low profile packages in which all of the package walls do not have the same height. In this case, the gel tends to wick up the higher walls and spill over the lower walls.
Thus, there is a need in the semiconductor device packaging art for a way to completely encapsulate the wire bonds and the chip without having the uncured gel run out of the package.
In some open cavity package designs, one or more components may even protrude above the top of the walls, in which case the lid would have to be adapted to accommodate the protrusion. However, the spilling problem renders it essentially impossible to use prior art encapsulation techniques to cover such protrusions.
Thus, there a need in the semiconductor packaging art for a way to completely encapsulate such protrusions.